Continuity mistake: In the beginning of Luke and Yoda's conversation, there's a cauldron hanging over the flames in the fireplace. In the shots facing Yoda, it is hanging higher up than in the previous shots. (00:28:30)
Continuity mistake: When Jabba pulls Leia's chain, there is a long shot as she leaps up onto his throne, and he is holding his wine goblet in his left hand, and pulling her chain with his right. However, in the very next shot (in which he says "Soon you will learn to appreciate me"), the entire frame is reversed. This is evident in Jabba's holding the wine in his "right" hand, his higher-placed nostril (which is normally on the left on his face) is now on the "right," Leia's reversal of her armlet, etc. (00:29:20)
Continuity mistake: When Luke is to be executed, he walks to the edge of the plank, and his feet are side by side as he stops walking. But in the beginning of the next shot, one of his feet is behind the other and we see him take another step. (00:30:55)
Continuity mistake: When Luke is prodded out to 'walk the plank', into the Sarlaac pit, he jumps off the board and springs himself back up. If you watch this carefully, you can see that only one hand catches the board and that just by the fingertips, but in the next shot both hands are springing him upwards. (00:31:20)
Continuity mistake: When the prisoner skiff is struck by laserfire, Han Solo is knocked overboard. A long shot shows him falling over sideways, yet when it changes to above the Sarlacc he is pointing straight downwards with his feet caught in the metalwork. (00:33:10)
Continuity mistake: When Boba Fett falls into the mouth of the Sarlaac, he disappears just before the beak touches him. (00:33:45)
Continuity mistake: When the Sarlacc tries to drag Lando into the pit, the tentacle is grabbing his ankle in some shots. In other shots, it is wrapped around his thigh as well. (00:33:50)
Continuity mistake: When Luke and Leia land on the skiff after swinging on the rope, Luke's facial hair is much darker than in the rest of the scene. (00:35:00)
Continuity mistake: On Jabba's sail barge, Luke gets shot in his mechanical hand. This leaves a large burn mark on the back of the hand, but when he swings to the skiff on the rope with Leia, the burn is gone. (00:36:00)
Continuity mistake: At the end of the scene when the Emperor and Darth Vader walks through the Death Star's hangar, you can see that the Emperor walks past a squad of grey suited commanders. But in the next shot there are only white dressed storm troopers and scout troopers on the right side of the Emperor. And if you compare the last shot in the scene with the earlier shots you can see differences in the soldiers' placements. (00:37:45)
Continuity mistake: When Han tells Lando that he can fly the Millennium Falcon during the battle, when Lando says, "Would you get going, you pirate", his entire outfit is reversed - his shoulder holster strap goes the other way, and his rank insignia changes side too. (00:50:00)
Continuity mistake: When Han enters the cockpit of the shuttle Tyderium, as the shot ends, he is about to walk past Luke. In the next shot, he has already walked past him and has begun to sit down. (00:50:25)
Continuity mistake: When Lando says goodbye to Han, after Ackbar's speech, Lando's lapel pin/medal swiches shoulders several times. (00:51:40)
Continuity mistake: After the Death Star explodes, in the first shot of the Ewoks, a grey one is standing next to Chewbacca (who can be seen on the extreme right). In the next shot, this Ewok has disappeared, and another one is standing next to C-3PO. This Ewok has disappeared in the following shot, and a third one, who was nowhere to be seen in the previous shot, is now standing in front of Chewbacca. Also, C-3PO, Chewbacca and the Rebel next to him are standing in different positions between shots. (00:58:15)
Continuity mistake: When Leia wakes up in the woods, she sits straight up. In the next shot, she is still lying on the ground. This is best seen in the widescreen version. (01:00:25)
Continuity mistake: When Leia offers the Ewok food, she holds it out in her left hand, but from a different angle it's in her right. (01:01:15)
Continuity mistake: When Luke is riding the speeder bike against the trooper, the black glove on his right hand jumps to his left and then back again. (01:01:15)
Continuity mistake: When Wicket sits down next to Leia on the log, he's holding his spear straight up. In the next shot, it is pointing downwards. (01:01:35)
Continuity mistake: After Vader enters the Emperor's throne room to tell him that Luke is one of the Rebels landing on Endor, as the shot ends, he is just about to walk up the stairs. In the next shot, he has already walked up the stairs. (01:03:50)
Continuity mistake: In the scene where Luke and the others are trapped in the net, the dead animal changes appearance (both injuries and actual appearance) and position several times. For example, the animal seen when they are in the net has a different mouth and a bigger area of white fur from the one seen a few shots earlier. (01:08:35)
Answer: Darth: "Obi-Wan once thought as you do. You don't know the power of the dark side. I must obey my master." It's implied that the dark side is intoxicating, once you totally give in to the dark side it has a hold on you, and appeals to morality and rationalization are useless against your lust for its power. (On a symbolic level, the dark side is a metaphor for vice. Darth Vader is an addict and abusive parent. It's actually funny how many scenes still make sense if you replace "the dark side" with "the bottle" or similar).
TonyPH
If we bring the prequels into it, it's one of the criticisms of those films that they only make the question of how much Anakin is a "true believer" more confusing. But it stands to reason that at first Anakin may feel vindicated in his resentment toward the Jedi. Later on, Vader may not feel that as strongly, but by then his anger has turned toward himself for failing to save Padme. He may feel that a man as terrible as he does not deserve to be "rescued" from the dark side, leading to a feedback loop where he only gets further enamored with its power and does more evil things which causes him to hate himself even more, and so it goes.
TonyPH